Fifteen uncoupled simple pendulums of monotonically increasing lengths dance together to produce visual traveling waves, standing waves, beating, and (seemingly) random motion - from NatSciDemos on youtube
A pendulum is made up of a weight suspended on a rod, string or wire. When the weight or bob is moved and let go, the pendulum will swing back and forth in a regular periodic motion. The affect of gravity on the bob results in a very predictable periodic motion. The length of the string/rod determines the frequency of its swing - Pendulums have been used in clocks for hundreds of years, because the motion is so regular.
Here's a pretty awesome UV version from brusspup on youtube (who has some other cool videos)